1. Field of the Invention
The present invention relates to an in-cylinder direct-injection spark-ignition engine with an electronic concentrated engine control system, employing an electronically-controlled fuel-injection system used to switch between a homogeneous combustion mode and a stratified combustion mode, and a variable valve timing and/or lift device used to be changeable a valve overlap during which both exhaust and intake valves are open in the same engine cylinder.
2. Description of the Prior Art
In recent years, there have been proposed and developed various in-cylinder direct-injection spark-ignition engines in which fuel is injected directly into the engine cylinder. Generally, on such direct-injection spark-ignition engines, a combustion mode is switchable between a homogeneous combustion mode and a stratified combustion mode, depending on engine operating conditions, such as engine speed and load. In more detail, the direct-injection spark-ignition engine uses two combustion modes, namely an early injection combustion mode (or a homogeneous combustion mode) where fuel-injection early in the intake stroke produces a homogeneous air-fuel mixture, and a late injection combustion mode (or a stratified combustion mode) where late fuel-injection delays the event until near the end of the compression stroke to produce a stratified air-fuel mixture. One such cylinder direct-injection spark-ignition engine (simply a DI engine) has been disclosed in Japanese Patent Provisional Publication No. 4-183945. The Japanese Patent Provisional Publication No. 4-183945 teaches selecting a stratified combustion mode from at least two combustion modes as previously discussed, during engine operation at partial loads, and also teaches increasing a valve overlap, during which the open periods of the intake and exhaust valves are overlapped, by means of a variable valve timing control device located on at least one of intake and exhaust valves during the stratified combustion mode. The increased valve overlap in the stratified combustion mode results in an increase in a so-called internal exhaust-gas recirculation, thus suppressing a temperature rise in the exhaust temperature (or the combustion temperature). This contributes to reduction in exhaust-emission levels of nitrogen oxides (NO.sub.x). In this conventional electronic engine control system of the direct-injection spark-ignition engine disclosed in the Japanese Patent Provisional Publication No. 4-183945, the valve overlap is set at a comparatively large valve overlap during the stratified combustion mode (or during partial loads), whereas the valve overlap is set at a comparatively small valve overlap during the homogeneous combustion mode (or during high loads). The prior art control system suffers from the following drawbacks. There is the response-time delay in increasing the actual valve overlap toward the desired small valve overlap, due to a slight response delay in valve-timing adjustment of the variable valve timing control device, even when the valve overlap is switched to the desired small valve overlap quickly at the time when the combustion mode has been completely switched to the homogeneous combustion mode. In this case, the actual valve overlap remains large for a while, just after the homogeneous combustion mode begins. This deteriorates combustion. To eliminate the response delay of the variable valve timing control device, suppose that the switch timing to the desired valve overlap is advanced evenly irrespective of during two different switch time periods, namely during a first switch time period from stratified to homogeneous combustion, and during a second switch time period from homogeneous to stratified combustion. During the transient switch time period from the homogeneous combustion mode (requiring a small valve overlap) to the stratified combustion mode (requiring a large valve overlap), there is the possibility that the actual valve overlap becomes large before the homogeneous combustion mode terminates. This also deteriorates combustion.